Ballistic missile defense has become a significant priority as national intelligence indicates a growing missile threat from rogue nations that might obtain weapons of mass destruction and use ballistic missiles to fire them at U.S. forces abroad, U.S. allies or the continental United States. A desirable engagement strategy against ballistic missiles is to intercept the target as early as possible during the boost phase or early ascent phase when the target is a large object and has not dispersed counter measures or multiple warheads. Such a strategy minimizes the requirements for warhead and decoy discrimination, and allows for multi-layered defense opportunities. A missile defense system supporting this strategy must include an accurate boost phase target state estimator. Without accurate target state estimates, a fire control system (also known throughout this document as weapons control system) cannot obtain valid intercept solutions for launching the interceptor during boost, and intercepting during the early ascent phase.
Challenges in developing boost phase tracking include unpredictable target accelerations while the target is in powered flight, and uncertainty of its burn time. Targets powered by solid rocket motor targets present the additional challenge of irregular thrust acceleration profiles. Due to the significant changes in acceleration during the target's boost phase, filters that assume constant acceleration cannot meet the stringent accuracies required of most fire control systems. Current state-of-the-art template-based filters use position and velocity templates assuming constant accelerations or rocket equation acceleration modeling. Such templates are subject to error attributable to motor burn variations, energy management (lofted/depressed trajectories), ISP variations, and early thrust terminations. FIG. 1 is a plot of target thrust versus time for “hot,” nominal and “cold” rocket motors or engines. The nominal thrust profile as a function of time after launch (TAL) is shown by plot 12. The actual motor may be a variation about this nominal profile. For a hot motor, the motor gain is greater than 1, K>1, with the thrust profile shown by plot 14. For a cold motor, the motor gain is less than 1, K<1, with the thrust profile shown by plot 16. It will be clear that the accelerations represented by the thrust profiles of FIG. 1 are significantly different.
In addition, uncertain knowledge of the time of launch of the target results in error in estimating the time after launch. The uncertainty in knowledge of the time after launch leads to error in indexing into the template, which in turn tends to introduce more error in the acceleration estimate. For example, if the estimate of time after launch in FIG. 1 is indicated by testimate and the actual time is indicated by tactual, there will be a substantial error in estimating the acceleration even if the proper plot were known. The indexing error and acceleration variations from the nominal template all contribute to erroneous acceleration estimates and ultimately poor estimates of the target velocity and position. The indexing error, delT, is the uncertainty in the target's time after launch (TAL),i.e., the time difference between the initial index into the nominal template and the target's true TAL.
Improved or alternative target tracking and intercept control are desired.